New materials for battery applications are in demand to meet the requirements of the next generation of technologies in portable electronics (laptop computers, cell phones, etc.), medical devices, and transportation. Rechargeable Li-ion batteries have enabled several types of consumer electronics to become more powerful while shrinking in size. The underlying goal that has driven materials research in this field is to use lower cost materials while delivering to the marketplace miniaturized, long-life batteries with high performance in terms of mass and volumetric energy densities.
In the development of Li-ion cells, significant advances in intercalation materials have occurred with the realization that oxides give higher capacities than previously utilized sulfides and also high cell voltages. Despite some twenty-plus years of research since the discovery of Li-ion insertion/deinsertion in layered sulfides (or chalcogenides in general), only a limited number of compounds have been employed in lithium battery devices. These include the layered-type LiCoO2 and derivatives based on LiMn1/3Ni1/3Co1/3O2, the 3D spinel LiMn2O4 compounds, and olivine LiFePO4 phase.
It has been recognized that transition metal (M) polyanion (XO4n−) structures, built from sharing vertex oxygen atoms of MOx polyhedra and XO4 tetrahedra anions (where X is S, P, or As), offer interesting possibilities. By altering the nature of X, the ionic-covalent character of the M-O bonding can be changed, with the changes attributed to an inductive effect (FIG. 1). Using this information, it has been possible to systematically map and tune transition-metal (TM) redox potentials into the desired high-potential regime. For instance, in the case of the Fe3+/Fe2+ redox couples in oxide-based materials, the potential (1.23 V vs Li/Li+) is too close to the Li/Li+ couple, which results in a low cell voltage. However, in the olivine LiFePO4 phase, with the use of the phosphate polyanion PO43−, the Fe3+/Fe2+ redox couples lie at higher potentials than in the oxide form (FIG. 1). With the incorporation of polyanions in the framework formation, the TM redox couples are stabilized, i.e., the potentials are made more positive in favor of cathode applications. These polyanions exhibit strong polarization of the oxygen atoms toward the X cation and subsequently the covalent component of the M-O bond is diminished by the inductive effect. Thus, the reduction potential of Fe3+/Fe2+ in state-of-the-art olivine LiFePO4 phase can reach as high as 3.5V vs Li/Li+.
Olivine LiFePO4 is an ionic conductor in which Li+-ion are transported through a pseudo-one-dimensional channel structure. The lithium iron(II) phosphate LiFePO4 phase has the ordered M′MPO4 olivine structure. Iron (M=Fe) is located in the middle of a slightly distorted FeO6 octahedron, and, as shown in FIG. 2, the FeO6 octahedra share corner oxygen atoms to form zig-zag planes with a Fe—O average bond-length higher than expected for an octahedrally coordinated iron in the +2 valence state. Lithium (M′=Li) is located in a second set of octahedral sites but distributed differently: LiO6 octahedra (not shown for clarity) share edges in order to form LiO6 chains running parallel to [001], the channel direction, which generates preferential rapid one-dimensional Li+-ion conductivity. With Li in a continuous chain of edge-shared octahedra on alternate ac planes, a reversible extraction/insertion of lithium from/into these chains would appear to be analogous to the two-dimensional extraction or insertion of lithium in the LiMO2 layered oxides with M=Co, Ni. It is noted that the PO4 tetrahedra bridge adjacent Fe planes in the olivine structure, which constrains the Li+-ion transport pathway; only the Li—O bonding confines the spacing between MO2 layers in the LiMO2 compounds.
Olivine LiFePO4, along with other members of the phospho-olivines LiMPO4 (M=Fe, Mn, Co), have now been extensively studied as positive electrode materials for rechargeable lithium batteries. Unlike mixed-metal oxides, polyanion-based compounds such as those of the phospho-olivines are intrinsic electronic insulators because these compounds structurally adopt a mixed framework that is composed of interlinked MO6 (M=transition metal cation) octahedra with closed-shell, non-magnetic polyanions. Limitation of material performance due to poor electronic conductivity has been improved by material processing through carbon coating (at the expense of lowering especially the volumetric capacity) and miniaturization of nanoparticles.
Downsizing bulk samples into nanometer-size LiFePO4 particles can bypass the slow kinetics, conceivably due to shortened ion/electron diffusion pathways, to improve its low intrinsic electronic/ionic conductivity. However, miniaturized particles can potentially suffer extensive structure defects and cation vacancies, along with inevitable moisture sensitivity on high-surface area particles and low packing density (thus low volumetric capacity), eventually causing capacity fade on continuous cycling.
Recently, researchers have examined the replacement of O2− with more electronegative anions, such as F−, either by means of anion substitution (Ellis, et al., “A Multifunctional 3.5 V Iron-based Phosphate Cathode for Rechargeable Batteries,” Nat. Mater. 2007, 6, 749-753) or direct fluorination (Al-Mamouri, et al., “Synthesis and Structure of the Calcium Copper Oxyfluoride, Ca2CuO2F2+δ,” J. Mater. Chem. 1995, 5 (6), 913-916) to increase the cell voltage. While anion substitution may result in a structurally distinct host electrode, direct fluorination offers an additional advantage in tuning the redox center by introducing a light, more electronegative anion and maintaining original ion conduction pathways in the host structure. Nevertheless, one of the main drawbacks with using these fluorine-substituted materials is their poor electronic conductivity.
Additionally, direct Li+-ion exchange from single crystals of certain new Fe(II,III)-containing phosphate compounds has been demonstrated (Becht, et al., Chem. Mater. 2006, 18, 4221-4223) showing facile ion-transport properties of open-framework solids. The parent compound, for example, was iron(III) phosphate Cs4.65(3)K4.35(1)Fe7(PO4)10. Single crystals of the parent compound were immersed in ANO3 solutions (A=Li, Na, K, Rb, Cs) and heated to encourage direct ion exchange. The formed plate-like crystals suffered microscopic damage as shown by apparent grooves seen on the surface of crystals. Subsequently, it was found that the structural solution was relatively poor.
While the above describes progress in the art, room for additional improvement exists.